Gradle Witness

A gradle plugin that enables static verification for remote dependencies.

Build systems like gradle and maven allow one to specify dependencies for versioned artifacts. An Android project might list dependencies like this:

dependency {
    compile 'com.actionbarsherlock:actionbarsherlock:4.4.0@aar'
    compile 'com.android.support:support-v4:19.0.1'
    compile 'com.google.android.gcm:gcm-client:1.0.2'
    compile 'se.emilsjolander:stickylistheaders:2.2.0'
}

This allows the sample Android project to very easily make use of versioned third party libraries like ActionBarSherlock, or StickyListHeaders. During the build process, gradle will automatically retrieve the libraries from the configured maven repositories and incorporate them into the build. This makes it easy to manage dependencies without having to check jars into a project's source tree.

Dependency Problems

A "published" maven/gradle artifact looks like this:

gson-2.2.4.jar
gson-2.2.4.jar.md5
gson-2.2.4.jar.sha1
gson-2.2.4.pom
gson-2.2.4.pom.md5
gson-2.2.4.pom.sha1

In the remote directory, the artifact consists of a POM file and a jar or aar, along with md5sum and sha1sum hash values for those files.

When gradle retrieves the artifact, it will also retrieve the md5sum and sha1sums to verify that they match the calculated md5sum and sha1sum of the retrieved files. The problem, obviously, is that if someone is able to compromise the remote maven repository and change the jar/aar for a dependency to include some malicious functionality, they could just as easily change the md5sum and sha1sum values the repository advertises as well.

The Witness Solution

This gradle plugin simply allows the author of a project to statically specify the sha256sum of the dependencies that it uses. For our dependency example above, gradle-witness would allow the project to specify:

dependency {
    compile 'com.actionbarsherlock:actionbarsherlock:4.4.0@aar'
    compile 'com.android.support:support-v4:19.0.1'
    compile 'com.google.android.gcm:gcm-client:1.0.2'
    compile 'se.emilsjolander:stickylistheaders:2.2.0'
}

dependencyVerification {
    verify = [
            'com.actionbarsherlock:actionbarsherlock:5ab04d74101f70024b222e3ff9c87bee151ec43331b4a2134b6cc08cf8565819',
            'com.android.support:support-v4:a4268abd6370c3fd3f94d2a7f9e6e755f5ddd62450cf8bbc62ba789e1274d585',
            'com.google.android.gcm:gcm-client:5ff578202f93dcba1c210d015deb4241c7cdad9b7867bd1b32e0a5f4c16986ca',
            'se.emilsjolander:stickylistheaders:89146b46c96fea0e40200474a2625cda10fe94891e4128f53cdb42375091b9b6',
    ]
}

The dependency definition is the same, but gradle-witness allows one to also specify a dependencyVerification definition as well. That definition should include a single list called verify with elements in the format of group_id:name:sha256sum.

At this point, running gradle build will first verify that all of the listed dependencies have the specified sha256sums. If there's a mismatch, the build is aborted. If the remote repository is later compromised, an attacker won't be able to undetectably modify these artifacts.

Using Witness

Unfortunately, it doesn't make sense to publish gradle-witness as an artifact, since that creates a bootstrapping problem. To use gradle-witness, the jar needs to be built and included in your project:

$ git clone https://github.com/WhisperSystems/gradle-witness.git
$ cd gradle-witness
$ gradle build
$ cp build/libs/gradle-witness.jar /path/to/your/project/libs/gradle-witness.jar

Then in your project's build.gradle, the buildscript needs to add a gradle-witness dependency. It might look something like:

buildscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:0.9.+'
        classpath files('libs/gradle-witness.jar')
    }
}

apply plugin: 'witness'

At this point you can use gradle-witness in your project. If you're feeling "trusting on first use," you can have gradle-witness calculate the sha256sum for all your project's dependencies (and transitive dependencies!) for you:

$ gradle -q calculateChecksums

This will print the full dependencyVerification definition to include in the project's build.gradle. For a project that has a dependency definition like:

dependency {
    compile 'com.actionbarsherlock:actionbarsherlock:4.4.0@aar'
    compile 'com.android.support:support-v4:19.0.1'
    compile 'com.google.android.gcm:gcm-client:1.0.2'
    compile 'se.emilsjolander:stickylistheaders:2.2.0'
}

Running gradle -q calculateChecksums will print:

dependencyVerification {
    verify = [
            'com.actionbarsherlock:actionbarsherlock:5ab04d74101f70024b222e3ff9c87bee151ec43331b4a2134b6cc08cf8565819',
            'com.android.support:support-v4:a4268abd6370c3fd3f94d2a7f9e6e755f5ddd62450cf8bbc62ba789e1274d585',
            'com.google.android.gcm:gcm-client:5ff578202f93dcba1c210d015deb4241c7cdad9b7867bd1b32e0a5f4c16986ca',
            'se.emilsjolander:stickylistheaders:89146b46c96fea0e40200474a2625cda10fe94891e4128f53cdb42375091b9b6',
    ]
}

...which you can then include directly below the dependency definition in the project's build.gradle.

And that's it! From then on, running a standard gradle build will verify the integrity of the project's dependencies.